All solid state Thin Film Batteries (TFB) are known to exhibit several advantages over conventional battery technology such as superior form factors, cycle life, power capability and safety. However, there is a need for cost effective and high-volume manufacturing (HVM) compatible fabrication technologies to enable broad market applicability of TFBs.
Most of the past and current state-of-the-art approaches, as they pertain to TFB and TFB fabrication technologies, have been conservative, wherein the efforts have been limited to scaling the basic technologies of the original Oak Ridge National Laboratory inventions that started in the early 1990s. More recently, some efforts to improve the properties and deposition rates for the cathode and electrolyte material layers have been seen. First is the application of a pulsed DC sputtering (i.e. pDC) technique to the cathode (LiCoO2 specifically; e.g. U.S. Patent Pub. 2006/0134522), with some improvement in deposition rate. In addition, substrate biasing has been applied to both cathode (U.S. Patent Pub. 2006/0134522 and U.S. Pat. No. 6,921,464, the second one with RF on the target) and electrolyte (U.S. Pat. No. 6,506,289) deposition steps, leading to some improved properties. However, much improvement is still needed.
FIGS. 1A to 1F illustrate a traditional process flow for fabricating a TFB on a substrate. In the figures, a top view is shown on the left, and a corresponding cross-section A-A is shown on the right. There are also other variations, e.g., an “inverted” structure, wherein the anode side is grown first, which are not illustrated here.
As shown in FIGS. 1A and 1B, processing begins by forming the cathode current collector (CCC) 102 and anode current collector (ACC) 104 on a substrate 100. This can be done by (pulsed) DC sputtering of metal targets (˜300 nm) to form the layers (e.g. main group metals such as Cu, Ag, Pd, Pt and Au, metal alloys, metalloids or carbon black), followed by masking and patterning for each of the CCC and ACC structures. It should be noted that if a metallic substrate is used, then the first layer may be a “patterned dielectric” deposited after a blanket CCC 102 (the CCC may be needed to block Li in the cathode from reacting with the substrate).
Next, in FIGS. 1C and 1D, the cathode 106 and electrolyte layers 108 are formed, respectively. RF sputtering has been the traditional method for depositing the cathode layer 106 (e.g. LiCoO2) and electrolyte layer 108 (e.g. Li3PO4 in N2). However, pulsed DC has been used for LiCoO2 deposition. The cathode 106 layer can be about 3 μm thick, and the electrolyte 108 layer can be about 1-2 μm thick.
Finally, in FIGS. 1E and 1F, the Li layer 110 and protective coating (PC) layer 112 are formed, respectively. The Li layer 110 can be formed using an evaporation process. The Li layer 110 can be about 3 μm thick (or other thickness depending on the thickness of the cathode layer) and the PC layer 112 can be in the range of 3 to 5 μm. The PC layer 112 can be a multilayer of parylene, metal or dielectric as disclosed by Oak Ridge National Laboratory. Note that, between formation of the Li layer and the PC layer, the part must be kept in an inert environment, such as argon gas.
There may be an additional “barrier” layer deposition step, prior to the CCC 102, if the CCC does not function as the barrier and if the substrate and patterning/architecture call for such a barrier layer. Also, the protective coating need not be a vacuum deposition step.
In typical processes, annealing of the cathode layer 106 will be required if the TFB performance specification calls for “plateau of operating voltage” and high power capability. A summary of the TFB properties can be found in N. J. Dudney, Materials Science and Engineering B 116, (2005)245-249.
While some improvements have been made to the original ORNL approaches, there are many problems with the prior art fabrication processes for TFBs that prevent them from being compatible with cost effective and high-volume manufacturing (HVM), and thereby preclude broad market applicability of TFBs. For example, issues with the state-of-the-art thin film cathode and cathode deposition processes include: (1) a low deposition rate leading to low throughput and inefficient scaling (of economy) for cost reduction, (2) a need for a high temperature anneal for the crystalline phase, which adds to process complexity, low throughput and limitations on the choice of substrate materials, and (3) a higher electrical and ionic resistivity, which limits the thickness of the cathode and high power (in battery operation) application, as well as the applicable sputtering methodology and sputtering power (which determines deposition rate).
With respect to the electrolyte, RF sputtering does not provide a high deposition rate with good conformality for pinhole free deposition. The low deposition rate RF sputtering process affects the throughput while the low conformality affects yield. The electrolyte is the key layer that allows the TFB to function as an energy storage device. More particularly, electrolyte layers with very high electrical resistivity (>1×1014 ohm-cm), have been deposited using RF sputtering with rates up to ˜2 Å/sec. Recently, when electrolyte layers were deposited using a PECVD process, the deposition rates appear to be higher, and provide reasonable properties in the resulting films. However, the long term reliability (cycle life) appears less than that observed in TFBs produced with RF sputtered layers. This discrepancy can be attributed to reactions between the charge carriers (lithium) and the impurity inclusions that likely result, during the PECVD processing, from incomplete oxidation of the organic ligands of the volatile precursors. As such, improvement in this layer will lead to significant outcomes for the overall technology.
Accordingly, a need remains in the art for fabrication processes and technologies for TFBs that are compatible with cost effective and high-volume manufacturing (HVM), and thereby enable broad market applicability of TFBs.